The present invention refers to new or recapped treads for tires with radial carcass reinforcement intended for use on transport vehicles and more particularly "heavy vehicles" which are intended to make long trips at sustained speed.
Such conditions of travel require high inflation pressures which, measured cold, are greater than 5 bars.
In addition to the radial carcass reinforcement, these tires comprise a crown reinforcement formed of at least two superimposed plies of cables of little extensibility, preferably of steel, crossed from one ply to the next and forming angles of at most 40.degree. with the circumferential direction. The tread reinforcement may advantageously be supplemented, on the one hand, by two half-plies formed of cables of little extensibility which form an angle of between 45.degree. and 80.degree. with the circumferential direction and, on the other hand, by one or more plies formed of elastic cables placed radially outward of the two crown plies of cables of little extensibility, and forming a small angle with the circumferential direction.
The crown reinforcement is surmounted by a tread which generally comprises a plurality of circumferential grooves, which have traces of linear, broken or undulated lines. These circumferential grooves have either identical or different widths. In the latter case, certain wide grooves are combined on the same tread with narrower grooves, the width of which, measured on the tread with the tire mounted on its operating rim and inflated to its rated pressure, may be such that, under the effect of the load imposed on the tire, the two quasi-vertical walls of these grooves may come into contact over all or part of their height.
Such treads are described in French Patent 1 452 048. FIG. 1 of said patent shows a tread having four wide circumferential grooves, while FIGS. 2 and 3 show treads having two wide circumferential grooves which cooperate with three or four circumferential grooves of smaller width.
In order to improve the longitudinal adherence of said treads, it is known to provide for the presence of transverse grooves the average orientation of which is between 45.degree. and 80.degree. with respect to the circumferential direction of the tire. In the case of the "heavy vehicle" tires in question, these transverse grooves generally have small widths of less than 3 mm which therefore may be referred to as incisions. These incisions, the traces of which may be linear, a broken line or an undulated line, either assure the connection between two circumferential grooves or originate within a circumferential groove and debouch onto a single circumferential groove or do not debouch on any circumferential groove, as shown in French Patent 1 452 048. The incisions which debouch on the two grooves defining a rib are by far the most effective in adherence, but on the other hand give rise to a very special irregular form of wear. The wear appears and develops more rapidly on the surface located on the side of the trailing edge of the relief element defined by the two circumferential grooves and the two transverse incisions than on the surface located on the side of the other edge, that is to say, the leading edge.
By definition, the leading edge of a relief element is the edge which penetrates first into the surface of contact between the tire and the ground, the tire obviously having a given direction of rotation, and the trailing edge therefore penetrating into the contact surface after the leading edge.
This form of wear is all the more disturbing as it affects the relief elements located on the ribs of the tread which are between two circumferential grooves and more particularly the ribs defined in the axial direction by two grooves of different width.